Field of Invention
The present invention relates to rectifiers. More particularly, the present invention relates to three-level rectifiers.
Description of Related Art
Medium or high voltage inverters configured in fans, pumps, compressors, and rolling mills are widely used in large mining production, petrochemical, municipal water supply, iron and steel metallurgy, electric power and other industries for improving energy savings.
A rectifier unit is an important part of the medium or high voltage inverter. Diodes that are not controlled for rectification has a simple structure and reliable and low cost advantages, but it results in large network harmonic waves. The phase shifting and voltage transformation can be used to reduce these harmonic waves, but greatly increases the system cost. Three-level technology effectively reduces the voltage stress of switching devices, and improves the harmonic waves and electro-magnetic interference (EMI), but the increase of the number of the semiconductor switching devices makes the control more complex.
Power semiconductor devices such as insulated gate bipolar transistors (IGBT) are widely used in the field of the medium or high voltage inverters. However, the voltage level of the high-voltage power semiconductor devices is limited, and the higher voltage level needs higher costs with poorer switching performance.
Low-voltage power semiconductor devices (e.g., IGBT) directly connected in series can effectively solve above problem. Due to the higher power and voltage levels, more power semiconductor switch modules and diode modules are connected in series to form each phase bridge arm of the conventional rectifier. In this way, the bridge arm has larger volume, and it is difficult to be installed and maintained; the stray inductance of a large commutation loop deteriorates the voltage stress of the switching devices and EMI of the system.